Increased Mass-Specific Maximal Fat Oxidation Rate with Small versus Large Muscle Mass Exercise

Abstract

Introduction: Skeletal muscle perfusion and oxygen (O2) delivery are restricted during whole-body exercise because of a limited cardiac output (Q˙). This study investigated the role of reducing central limitations to exercise on the maximal fat oxidation rate (MFO) by comparing mass-specific MFO (per kilogram of active lean mass) during one-legged (1L) and two-legged (2L) cycling. We hypothesized that the mass-specific MFO would be higher during 1L than 2L cycling.

Methods: Twelve male subjects (V̇O2peak, 59.3 ± 8.4 mL·kg-1·min-1; mean ± SD) performed step-incremental 2L- (30%-80% of V̇O2peak) and 1L (50% of 2L power output, i.e., equal power output per leg) cycling (counterbalanced) while steady-state pulmonary gas exchanges, Q˙ (pulse-contour analysis), and skeletal muscle (vastus lateralis) oxygenation (near-infrared spectroscopy) were determined. MFO and the associated power output (FatMax) were calculated from pulmonary gas exchanges and stoichiometric equations. A counterweight (10.9 kg) was added to the contralateral pedal arm during 1L cycling. Leg lean mass was determined by DEXA.

Results: The absolute MFO was 24% lower (0.31 ± 0.12 vs 0.44 ± 0.20 g·min-1, P = 0.018), whereas mass-specific MFO was 52% higher (28 ± 11 vs 20 ± 10 mg·min-1·kg-1, P = 0.009) during 1L than 2L cycling. FatMax was similar expressed as power output per leg (60 ± 28 vs 58 ± 22 W, P = 0.649). Q˙ increased more from rest to exercise during 1L than 2L cycling when expressed per active leg (ANOVA main effect: P = 0.003). Tissue oxygenation index and Δ[deoxy(Hb + Mb)] were not different between exercise modes (ANOVA main effects: P ≥ 0.587), indicating similar skeletal muscle fractional O2 extraction.

Conclusions: Mass-specific MFO is increased by exercising a small muscle mass, potentially explained by increased perfusion and more favorable conditions for O2 delivery than during whole-body exercise.